Hey guys
I tries all solutions here but non were working.
In the end i got into the sources of shadowbox and i noticed that the markup was not inserted in the DOM at the right moment, so the instruction "U.get('sb-container').style.display = 'block';" was failing (since sb-container was inexistant) and DOM Ready was not called right.
So i decided to switch to some other library, thinking that the ready event has got to do with the JS library used.
SOLUTION:
-->i replaced prototype.js with jquery.js.
-->i replaced document.observe with window.onload
-->IT WORKS in IE8, FF
Of course, not all of you can change so easily the JS library you use, since there may be other dependencies to consider.
Hope this helps,
Cata